The present invention relates to a Faraday rotation attenuator and, more particularly, to the use of multiple domain garnet, disposed between single mode transmission paths, to form an attenuator of variable strength by varying the external magnetic field applied to the ferromagnetic garnet.
Optical attenuators are useful in a number of different optical system applications. For example, attenuators are used in optical amplifier systems to "balance" the gain across the different operating wavelengths. An optical attenuator may also be inserted in the signal path of an amplifier system beyond the pre-amplifier location to stabilize the saturation level of the power amplifier Wavelength division multiplexed systems may also use optical attenuators in the receiver portion of the system to compensate for variations in received signal power
Most conventional prior art attenuators include a motorized arrangement, using a stepper motor to rotate one or more objects into and out of the optical signal path. Although such arrangements are adjustable and can provide the desired degree of attenuation, they are relatively slow and have reliability concerns related to the need to physically move the objects with respect to the signal path. A non-mechanical optical attenuator is described in the article entitled "Non-Mechanical Variable Attenuator Module using Faraday Effect", by N. Fukushima et al. appearing in OSA Trends in Optics and Photonics, 1996, Vol. 5, at pp. 249-52. In this arrangement, a variable Faraday rotator is disposed between a pair of polarizers. Two magnets are disposed to surround the magneto-optic crystal material in the rotator, a permanent magnet and an electromagnet. The permanent magnet thus defines a certain domain within the material and the polarizers control the polarization state of the input and output signals. The application of a current to the electromagnet is then used to control the degree of attenuation and has been found to provide attenuation in the range of 1.6 to 25 dB.
While the Fukishima et al. arrangement may be considered an advance over the mechanical attenuators, the arrangement is rather bulky, requiring the use of a pair of magnets, as well as the polarizers to control the state of the signals passing through the attenuator.
Thus, a need remains in the art for providing adjustable optical attenuator without the need to physically move system components to achieve the attenuation and which is less bulky than the prior art non-mechanical alternatives.